In the type of LAN to which the invention is applicable, there is a main information channel, which may be arranged in a bus configuration. The stations are connected in multidrop fashion to the main information channel, and the information is put onto the main channel in the form of discrete packets.
For a LAN to be suitable for Voice/Data integration, as in the invention, the LAN should be of the kind that incorporates a scheduling procedure. By means of the scheduling procedure, the LAN can be arranged to give each station in turn an opportunity to put Voice packets and Data packets onto the main channel.
In some LANs, the scheduling function is provided by sending the scheduling signals from station to station via the main channel. In other LANs, an extra scheduling channel is provided, which is separate from the main channel, for scheduling.
It is preferred, for the purposes of the invention, that the LAN is of the kind in which the scheduling is done on a separate scheduling channel. If the scheduling were to be done on the main channel, the cheudling signals would have to be intercalated with the information packets and consequently there would be a loss of capacity in the main channel.
The main channel is an expensive item, having the capability of transmitting hundreds of megabits per second. By providing a separate channel for scheduling, the capacity of this expensive channel can be fully utilized. The scheduling channel on the other hand can be quite inexpensive, because the scheduling channel need only transmit the scheduling information, not the packets of information.
It is recognised in the invention that the Welnet (Trademark) LAN System is especially suitable for the integration of Voice and Data. In the Welnet system, a separate scheduling channel is provided. The scheduling channel is in the form of a loop, and the scheduling signals are passed from station to station around the loop. Each station is capable of receiving a scheduling signal only from the station preceding it in the loop, and each station is capable of transmitting a scheduling signal only to the station after it in the loop.
The scheduling signals may be regarded as tokens. Only when a station has the correct token is the station allowed to put its information packet, either a Voice packet or a Data packet, if it has one ready, on to the main channel. Each station has the opportunity to send only one packet of information onto the main channel, when it has the token. The station must then wait until it receives another appropriate token before it can transmit a further packet of information onto the main channel.
In the Welnet system, a station (eg station J) transmits the token to the next station (station K) on the scheduling channel at the same time as station J puts its information packet onto the main channel. Station K, upon receipt of the token, and if it has an information packet to send, monitors the main channel (in fact each station monitors the main channel all the time) and when the main channel is clear, i.e. when the information packet from station J has finished, station K puts its information packet onto the main channel.
In the Welnet system, each station passes the token as soon as that station has started to transmit its packet on to the main information channel. Thus, in a Welnet LAN, the scheduling token and the information packet are sent out at the same time, and in parallel.
In the Welnet LAN, the scheduling channel has a much lower bit-transmission-rat capability than the main channel. A system with a separate scheduling channel, like the Welnet LAN System, would be to some extent uneconomical if the scheduling channel had to be as expensive as the main channel. It is recognized in the invention that, as a result of the relative crudeness of the scheduling channel, the tokens that pass around the scheduling channel should be kept very simple, i.e. very short. If it were to take an appreciable time to transmit the tokens--i.e. if the tokens were to consist themselves of a large number of bits--then the scheduling channel would have to be sophisticated.
It is recognized, in the invention, that the scheduling signals, ie. the token or tokens, therefore should occupy as few bits as possible. The invention lies in recognizing how the small number of bits that are thus available for the tokens can be used to ensure efficient and trouble-free scheduling of integrated Voice and Data services.